This invention relates to synthetic ester compositions and in particular to compositions based on a blend of poly(neopentyl) polyol ester and an ester of a polyol containing at least two hydroxyl groups, which is inhibited with a unique combination of additives. The compositions are particularly well suited for use as a coolant/lubricant in a rotary screw air compressor. The coolant/lubricant compositions minimize use of esters formed from highly odoriferous acids, yet provide extended life, higher temperature operation, excellent demulsibility, biodegradability, rust protection and hydrolytic stabilityxe2x80x94a combination of properties not available in existing synthetic compressor coolant/lubricant compositions.
It is well known to use hydrocarbon lubricating oils in rotary screw compressors. The oil seals the rotors, lubricates the bearings, cools the compressed gases and removes water condensed from the compressed gases. The high temperature and pressure and presence of water causes the hydrocarbon oils to break down and create sludge in a relatively short time.
Efforts to lengthen the useful life of coolants for air compressors have led to the use of synthetic esters as basestocks. Typical operating temperatures are between about 80xc2x0 to 104xc2x0 C. (170xc2x0 to 220xc2x0 F.). While these temperatures are not high for a synthetic lubricant, the environment is highly oxidative so that mineral oils tend to break down after about 1,000 hours of use and must be changed. Several of the synthetic coolant/lubricants presently in use extend the intervals between changing lubricants to as much as 8,000 hours.
A synthetic ester coolant based on diesters of adipic acid and phthalic acid have been used for more than 25 years in a variety of compressors including rotary screw compressors. Other synthetic coolants in use are based on synthetic hydrocarbons or poly alpha olefins (PAO""s), polyalkylene glycols (PAG""s), silicones, and mixtures of synthetic hydrocarbons and dibasic acid esters. All of these products provide extended lubricant life when compared to petroleum based coolants used in the highly oxidative environment in a screw compressor. However, none of the available lubricants are able to provide all of the desired properties, in particular the combination of biodegradability with hydrolytic stability, rust protection, and demulsibility while avoiding offensive odors.
In addition to these performance properties, certain physical properties are required, such as providing efficient cooling, low temperature start-up, sealing of the rotors, and lubrication of the bearings. In particular, the desirable physical properties include a viscosity at 100xc2x0 C. (212xc2x0 F.) of at least 8 cSt, a viscosity index of at least 140, a flash point of at least 260xc2x0 C. (500xc2x0 F.), and a pour point of at most xe2x88x9250xc2x0 C. (xe2x88x9258xc2x0 F.)
A wide operating temperature range for the coolant is highly desirable. In order to permit low temperature starting, a lubricant having a. pour point less than xe2x88x9250xc2x0 C. (xe2x88x9258xc2x0 F.) is required. Suitable high temperature viscosity properties and low volatility are needed to permit suitable lubrication at higher operating temperatures. Thus, the coolant composition should have a flash point above 260xc2x0 C. which is required by some manufacturers for safety reasons.
All screw compressor coolant/lubricants will come into contact with water condensed from the compressed air. Therefore, any lubricant must exhibit good hydrolytic stability and provide good rust and corrosion protection. It is an added advantage to provide a lubricant exhibiting good demulsibility. This will permit separation of water from the spent lubricant and facilitate disposal and recycling of the spent lubricant.
Increased biodegradability is also highly desirable. In addition, the elimination of heavy metals, utilized in some commercial coolants, is also desirable from an environmental point of view.
It is also highly desirable to avoid objectionable odors.
Thus, it becomes desirable to avoid or minimize the use of esters formed from carboxylic acids having less than seven Lo carbon atoms. Finally, adequate regulatory listing is desirable.
Existing petroleum based and synthetic air compressor coolant/lubricants are not capable of providing all of the desired performance and physical properties for modern rotary screw compressors. Often, modification of one component of a synthetic lubricant will improve a certain desired property, but at the expense of another. For example, coolants based on PAO""s have poor biodegradability; coolants based on adipate diesters cannot provide the viscosity, or flash point properties; coolants based on phthalate diester have poor biodegradability and viscosity indices; coolants based on silicones have poor biodegradability and rust protection; coolants based on polyalkylene glycol have poor demulsibility and inadequate rust protection; and coolants based on mixtures of PAO""s and diester do not provide the desired biodegradability.
The following table summarizes the desired properties generally available from current types of rotary screw compressor coolant/lubricants. While several different types may be noted as having satisfactory performance of a particular property, there will be variations within acceptable ranges, some of which may be significant to the user. An xe2x80x9cXxe2x80x9d indicates that the referenced type of commercially available coolant/lubricant can provide the desired property, and an xe2x80x9cOxe2x80x9d indicates that it is not fully acceptable.
One of the most widely used high performance rotary screw compressor coolants is a PAG/ester synthetic lubricant of the type described in U.S. Pat. No. 4,302,343 to Carswell et al. The Carswell PAG/ester lubricant is an inhibited blend of a polyalkylene glycol with hindered alkanoic ester of aliphatic polyhydric alcohols having three to eight hydroxyl groups and five to ten carbon atoms. When properly blended and mixed with additives, the resulting synthetic coolant/lubricant has been found to satisfy the high temperature viscosity requirements and exhibit stability to the heat, air and water environment. However, ferrous metal corrosion resistance and demulsibility has been found less than satisfactory. The presence of the heavy metal barium also presents disposal concerns.
U.S. Pat. No. 4,175,045 to Timony describes a compressor lubricant formed of a polyol ester of a carboxylic acid having from about four to thirteen carbon atoms. The polyols utilized are pentaerythritol, dipentaerythritol, trimethylolpropane, or combinations thereof. The lubricants are blends of the pentaerythritol ester and dipentaerythritol esters. While providing satisfactory useful life and temperature characteristics, the viscosity at 100xc2x0 C. (212xc2x0 F.) and pour point do not meet the desired values as set forth in Table I. In addition, the lubricants are based on substantial amounts of esters formed from valeric acid and thus in use present undesirable odor problems. Improvements in rust prevention and demulsibility are also needed.
U.S. Pat. No. 3,694,382 to Kleiman, Malek and Lonsker discloses an ester lubricant which is a blend of 1.5-2.5 parts of a trimethyolpropane ester of aliphatic monocarboxylic acids containing from 4-12 carbon atoms with 0.75-2 parts of a dipentaerythritol ester of a mixture aliphatic monocarboxylic acids containing 4-10 carbon atoms. It is disclosed that the dipentaerythritol may include minor amounts of monopentaerythritol and higher condensation products of pentaerythritol. The preferred blends include more than 50 weight percent dipentaerythritol ester. The products are suitable as synthetic lubricants in turbines. and turbojet engines.
High temperature compressor oils are described in U.S. Pat. No. 5,156,759 to Culpon Jr. The compressor oils include a polyalphaolefin base lubricating oil together with well-known additive compositions including amine antioxidants and rust inhibitors/metal passivators which are a triazole and/or alkenylsuccinic acid rust inhibitor. Lubricant additives are also shown in U.S. Pat. No. 2,830,019 to Fields and Brehm. These additives are reaction products of an amine and carboxylic acids for improving color stability and providing corrosion resistance. Additionally, aspartic acid derivatives as corrosion inhibitors of the type utilized herein are disclosed in Kugle and Blank, U.S. Pat. No. 5,275,749. Such compositions are utilized to improve demulsibility as well as anti-corrosion and anti-wear properties of the lubricant.
High temperature compressor oils are described in Culpon U.S. Pat. No. 5,156,759. The compressor oils include a polyolphaolefin base lubricating oil together with well-known additive compositions including amino antioxidants and rust inhibitor/metal passivator which is a triazole and/or alkenylsuccinic acid rust inhibitor. Lubricant additives are also shown in U.S. Pat. No. 2,830,019 to Fields and Brehm. The additives are reaction products of an amine and carboxylic acids for improving color stability and providing corrosion resistance. Additional aspartic acid esters as corrosion inhibitors of the type utilized herein are disclosed in Kugle and Blank, U.S. Pat. No. 5,275,749. Such compositions are utilized to improve demulsibility as well as anti-corrosion and anti-wear properties of the lubricant.
European Patent Application No. 498 152 of CPI Engineering provides lubricant compositions miscible in non-chlorinated, fluorinated hydrocarbon refrigerants. These lubricants are based on polyhydric alcohol esters of trimethylolpropane, mono-and/or dipentaerythritol ester made with branched carboxylic acids. Here, miscibility with the refrigerant composition is essential which requires the use of the branched esters.
As noted, the prior art describes a wide variety of synthetic lubricants based on various polyol esters and blends thereof admixed with various additives to improve performance. While these synthetic rotary screw air compressor coolant/lubricants are presently in wide use, it remains desirable to provide an improved coolant/lubricant composition which can provide all the significant desired properties noted in Table I, particularly increased oxidative stability, improved demulsibility, improved rust protection, increased biodegradability and low odor in use.
Generally speaking, in accordance with the invention, an improved synthetic coolant/lubricant composition for a rotary screw compressor providing superior performance based exclusively on a polyol ester base stock and suitable additives is provided. The ester portion of the composition is a blend of a major amount of poly(neopentyl polyol) ester and a minor amount of a polyol ester formed from a polyol having at least two hydroxyl groups. The composition includes antioxidants, yellow metal pacifiers, rust inhibitors, hydrolytic stability improver and may include an antifoam additive. This unique approach of using a poly (pentaerythritol) ester based preparation was found necessary to obtain all the desired performance properties, particularly the combination of extended life, low pour point, high flash point, good demulsibility and reduced odor. This approach differs from the esters described in the patent literature and synthetic coolant/lubricant compositions in use today.
The poly(neopentyl polyol) ester component is a poly(pentaerythritol) ester formed by esterifying partial esters of pentaerythritol, dipentaerythitol, tripentaerythritol, tetrapentaerythitol, etc. with at least one monocarboxylic acid or a mixture of acids having from about six to twelve carbon atoms. The poly(pentaerythitol) ester is formed by first reacting pentaerythitol with the selected carboxylic acid or a mixture of acids with an excess of hydroxyl groups to carboxyl groups to form the partial esters. In the preferred embodiments, the carboxylic acid is a liner acid having from seven to twelve carbon atoms.
The polyol ester component is utilized to balance the properties of the poly (pentaerythritol) ester, especially to lower the pour point and improve biodegradability without adversely affecting any other desired properties. It is prepared by reacting a hindered polyol having from five to eight carbon atoms and at least two hydroxyl groups with a monocarboxylic acid having from seven to twelve carbon atoms. In the preferred embodiments, the polyol is trimethylolpropane and the acid is a linear acid having from seven to twelve carbon atoms.
The coolant/lubricant composition is formed by mixing from 50 to 80 weight percent poly(neopentyl polyol) ester and 20 to 50 percent polyol ester, and adding effective amounts of additives, for example between 0.5 to 10 weight percent antioxidants, yellow metal pacifier, rust inhibitors and an antifoam agent.
Accordingly, it is the object of the invention to provide an improved rotary screw compressor coolant/lubricant.
Another object of the invention is to provide an improved rotary screw compressor coolant/lubricant of a blend of poly(neopentyl polyol) esters and traditional ester polyol.
A further object of the invention is to provide an improved rotary screw compressor coolant/lubricant having increased oxidative stability, improved demulsibility, improved rust protection, increased biodegradability, and reduced volatility compared to conventional synthetic compressor lubricants.
Yet another object of the invention is to provide an improved additive package designed to provide improved demulsibility, enhanced rust protection and improved hydrolytic stability for a synthetic ester rotary screw compressor coolant/lubricant.
Yet a further object of the invention is to provide a method of lubricating a rotary screw compressor with a coolant/lubricant composition based entirely on polyol esters.
Another object of the invention is to provide a synthetic ester rotary screw compressor coolant/lubricant meeting all the desired properties without including significant amounts of esters based on acids having unpleasant odors.
Still another object of the invention is to provide a method of lubricating a rotary screw compressor with a coolant/lubricant composition based on a blend of poly(neopentyl polyol) esters and polyol esters.
Still a further object of the invention is to provide a new additive package for a synthetic ester lubricant to improve rust protection, hydrolytic stability and demulsibility.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises a composition of matter possessing the characteristics, properties, and the relation of components which will be exemplified in the composition hereinafter described, and the scope of the invention will be indicated in the claims.
The polyol ester based compressor coolant/lubricant compositions prepared in accordance with the invention include a polyol ester blend and performance additives designed to provide improved demulsibility, biodegradability, provide excellent ferrous metal rust prevention and improved hydrolytic stability. There are no odoriferous heavy components or metals in the composition.
The synthetic ester portion of the lubricant is a blend of poly(neopentyl polyol) ester and polyol ester. The lubricant includes from 50-80 weight percent of the poly(neopentyl polyol) ester and from 20 to 50 weight percent polyol ester. Preferably, the poly(neopentyl polyol) ester is present between 55 to 75 weight percent, and in the most preferred embodiments, between 65-70 weight percent, based on the total weight of the composition with the balance the polyol ester and the additives.
Preferably, the poly(neopentyl polyol) ester , which is a mixture of esters of pentaerythritol, dipentaerythritol, tripentaerythritol, tetrapentaerythritol, etc. formed by reacting pentaerythritol with at least one monocarboxylic acid having from about 6 to 12 carbon atoms in the presence of an excess of hydroxyl groups to carboxyl groups. Most preferably, the acids have from 7 to 10 carbon atoms and are linear. In the most preferred aspects of the invention, the acid component of the poly(neopentyl polyol) ester is a linear monocarboxylic acid, or a mixture of linear acids with about five weight percent or less branched acid.
Acids having less than six carbon atoms are not within the scope of the invention, because they may have impart objectionable odors in use. Suitable acids include, but are not limited to oenanthic acid, caprylic acid, pelargonic acid and capric acid. Preferably, the straight chain acid is a mixture of heptanoic (C7) and caprylic-capric (C8-C10). The caprylic-capric acid is usually identified as being a mixture of 8 and 10 carbon acids, but actually includes C6 acid (less than about 5 weight and). Use of linear acids to prepare the esters adds to the biodegradability and viscosity index of the coolant/lubricant composition.
The initial stage of the reaction to form the poly (neopentyl polyol) ester is conducted in the manner described by Leibfried in U.S. Pat. No. 3,670,013. Here, a reaction mixture of pentaerythriyol (272 w) and valeric acid (217 v) was placed into a reactor with extra valeric acid (38 v) in a condenser to assure a constant level of valeric acid in the reaction mixture. The mixture was heated to a temperature of 171xc2x0 C. and concentrated sulfuric acid (1.0 w) diluted with water (2 v) was added. The reaction mixture was heated to 192xc2x0 C. and maintained until 50.5 v of water was removed after about 1.4 hours. His analysis of the product was pentaerythritol, dipentherythritol, tripentaerythritol and tetrapentaerythritol at weight ratios of 34:38:19:8.
The neopentyl polyol and selected acid or acid mixture are mixed in the presence of a strong acid catalyst and heated. The reaction is continued until the desired viscosity of the reaction mixture is reached. At this point when the starting neopentyl polyol is pentaerythritol, the mixture includes partial esters of pentaerythritol, dipentaerythritol, tripentaerythritol, tetrapentaerythritol and the like. In order to complete the esterification of the partial esters, an excess of the acid is added to the reaction mixture which is then heated, water of reaction removed and acid returned to the reactor.
In the preferred embodiment the invention, the poly(neopentyl polyol) partial ester is poly(pentaerythitol) formed from pentaerythitol. The poly(pentaerythitol) is prepared by introducing a reaction mixture of pentaerythitol and a linear monocarboxylic acid having from 6 to 12 carbon atoms in an initial mole ratio of carboxyl groups to hydroxyl groups of about 0.25:1 to about 0.5:1 and an effective amount of an acid catalyst material into a reaction zone as described in Liebfried.
The acid catalyst is at least one acid esterification catalyst. Examples of acid esterification catalysts include mineral acids, preferably, sulfuric acid, hydrochloric acid, and the like, acid salts such as, for example, sodium bisulfate, sodium bisulfite, and the like, sulfonic acids such as, for example, benzene sulfonic acid, toluene sulfonic acid, polystyrene sulfonic acid, methyl sulfonic acid, ethyl sulfonic acid, and the like. The reaction mixture is heated to between about 150xc2x0 and 200xc2x0 C. while withdrawing acid vapor and water vapor to yield the poly(pentaerythritol) partial ester product.
Prior to esterifing, the partial ester product will include a variety of condensation products of pentaerythritol. The mixture will include significantly more dipentaerythritol than the 10 to 15 weight percent generally present in commercially available dipentaerythritol. Depending on the initial ratio of carboxyl groups to hydroxyl groups and selection of reaction conditions, the partial ester product may include the following components in the weight ranges specified in the following table.
The amount of the preferred heptanoic and caprylic-capric acid mixture for preparing the poly(neopentyl polyol) ester may vary widely. Initially, an excess of hydroxyl groups to carboxylic acid groups is present to form the partial esters of pentaerythritol, dipentaerythritol, tripentaerythritol, tetrapentaerythritol, etc. The excess of hydroxyl groups is necessary to promote the condensation of the polyol into partial esters during the reaction. The molar ratio of acid mixture to the polyol can be varied depending on the desired degree of condensation and the ultimate desired viscosity of the lubricant. After formation of the partial esters, generally, a 10 to 25 percent excess of the mixture of heptanoic acid and C8-C10 acid is added to the reactor vessel and heated. Water of reaction is collected during the reaction while the acids are returned to the reactor. Presence of a vacuum will facilitate the reaction. When the hydroxyl value is reduced to a sufficiently low level, the bulk of the excess acid is removed by vacuum distillation. Any residual acidity is neutralized with an alkali. The resulting poly(neopentyl polyol) ester is dried and filtered.
The polyol ester preferably is an ester of a polyol having between 5 and 8 carbon atoms and at least two hydroxyl groups and a linear monocarboxylic acid having from 6 to 12 carbon atoms. Specific examples of polyol useful in the present invention include neopentyl glycol, pentaerythitol, dipentaerythitol, tripentaerythitol, trimethylolpropane, trimethylolethane etc. Mixtures of any of the above polyol can be utilized.
The acid component of the polyol ester can be the same or vary from that utilized to prepare the poly(neopentyl polyol) ester. Accordingly, monocarboxylic acids having between 7 to 12 carbon atoms, such as heptanoic (C7) and caprylic-capric (C8-C10) with minimal C6 and lower acids are preferred. In the most preferred embodiments, a linear acid, namely caprylic-capric acid is utilized to form the polyol ester. The amount of acid present in the reaction mixture can vary widely. Since the desire is to completely esterify the polyol , an excess generally in the amount of between 10 to 25 percent excess acid relative to the stoichiometric amount is added.
The polyol ester is formed by reacting the polyol with an excess of the carboxylic acid, removing the water of reaction and returning the unreacted acid to the heated reactor vessel. The reactor is equipped with a mechanical stirrer, thermocouple, thermoregulator, Dean Stark trap, condenser, nitrogen sparger and vacuum source. The esterification may or may not be carried out in the presence of an esterification catalyst, which are well known in the art, such as stannous oxalate. The polyol is preferably trimethylolpropane which is esterified with a C8-C10 acid to form the desired triester. The trimethylolpropane-C8-C10 triester is blended with the poly(pentaerythritol) esters to form the lubricant so that the triester is present between 20 to 50 weight percent of the lubricant, preferably between 25 to 40 weight percent. In the preferred embodiment 30 to 35 weight is percent is used.
The polyol, and the monocarboxylic acid in excess of 10 to 15 percent, are charged to the reactor vessel. The vessel is heated and water of reaction is collected in the trap during the reaction. The acids are returned to the reactor. Vacuum is applied to maintain the reaction. When the hydroxyl value is reduced to a sufficiently low level, the bulk of the excess acid is removed by vacuum distillation. The residual acidity is neutralized with an alkali. Finally, the resulting polyol ester product is dried and filtered.
In order to improve the properties of the (polyneopentyl polyol) based ester compositions prepared in accordance with the invention as coolant/lubricants, effective amounts of various additives are added. For example, the oxidative stability of the ester based coolant/lubricant can be improved by adding an effective amount of at least one antioxidant. Examples of suitable antioxidants which can be used are secondary arylamines, and phenyl naphthylamines, i.e. both alpha and beta-naphthyl amines; diphenyl amine; iminodibenzyl; p, p-dioctyl-diphenylamine; and related aromatic amines. Other suitable antioxidants are hindered phenolics, such as 2-t-butylphenol, 2,6-di-t-butylphenol and 4-methyl-2,6-di-t-butylphenol and the like.
Generally, between 0.5 to 10 weight percent antioxidant is included in the ester composition. Preferably, between 1 to 5 weight percent is included. The preferred antioxidant is a mixture of secondary arylamines, such as dioctyldiphenyl amine and phenylalphanaphthyl amine. When these two latter amines are utilized in combination, between 0.01 to 5 weight percent of each is included, and preferably between 0.1 to 0.5 weight percent of each is included.
A rust inhibitor is included to inhibit ferrous metal corrosion which is a serious problem due to the condensation of water from the compressed gases. Rust inhibitors have traditionally been compounds containing heavy metals which are desirable to avoid. However, a combination of amino acid derivatives with substituted primary and secondary amines have been found to provide a synergistic effectxe2x80x94not only providing improved ferrous metal rust prevention and improved hydrolytic stability, but doing this without adversely affecting demulsibility. The amino acid derivative found suitable is available as KCORR-100E (50% active) from King Industries which is an N-acyl-N- alkoxyalkyl substituted amino acid ester and a mixture of substituted primary and secondary amines known as Duomeen TDO and Ethomeen T-12 from Akzo Nobel. These rust inhibitors are utilized in combination to maintain good demulsibility, and provide excellent rust prevention and improved hydrolytic stability.
The amino acid derivative and substituted amines and diamines present as rust inhibitors are included in an amount between 0.05 to 10 weight percent of the lubricant. Preferably, between 0.1 to 6 weight percent is included. In the preferred embodiments, a total of 0.50 weight percent of the rust inhibitor, such as KCORR-100E, Ethomeen T-12 and Duomeen TDO, are included.
Cuprous metal deactivators, known as yellow metal pacifiers, may also be included. Examples are imidazole, benzimidazole, pyrazole, benzotriazole, tolyltriazole, 2-methyl benzimidazole, 3,5-dimethylpyrazole and methylene bis-benzotriazole. Preferably, an aryltriazole, such as tolyltriazole is utilized. Such a yellow metal pacifier is included in an effective amount, generally 0.001 to 0.5 weight percent of the lubricant. Preferably, between 0.01 to 0.2 weight percent is included. In the preferred embodiment, about 0.05 weight percent tolyltriazole is utilized as a yellow metal pacifier.
In addition to these antioxidants, yellow metal pacifiers and rust inhibitors, it may be desirable to include an effective amount of a defoamer which will prevent undesirable foaming of the lubricant as it is worked between the screws of the compressor. A silicone fluid present in a minor amount is effective. Generally, 0.001-10 parts per million of defoamer is sufficient. In the preferred embodiment, 1 ppm is utilized.
The lubricant is formed by placing the desired amounts of the poly(neopentyl polyol) ester and the polyol ester blend in a vessel equipped with a mechanical agitator, thermocouple, thermoregulator and nitrogen sparge. The mixture is heated to approximately 100xc2x0 C. (212xc2x0 F.). At this time, the antioxidants and yellow metal pacifier additives are added and agitated until dissolved. The mixture is cooled to less than 50xc2x0 C. (122xc2x0 F.) and the amino acid derivative, and mixture of substituted primary and secondary amines are added. The mixture is agitated, filtered and then defoamer added.
The ester coolant/lubricants prepared in accordance with the invention are specifically designed to be utilized in a rotary screw air compressor. Accordingly, it is designed to have a viscosity in the range of 5 to 15 centistokes at 100xc2x0 C. (212xc2x0 F.) and preferably 7 to 10 centistokes at 100xc2x0 C. (212xc2x0 F.) and a pour point in the range of xe2x88x9229xc2x0 to xe2x88x9254xc2x0 C. (xe2x88x9220xc2x0 to xe2x88x9265xc2x0 F.).